Straight from the DigiExpo who have the latest code of GT5, the confirmation of track editing in GT5 is at 2:25 when it is said "you can make your own tracks". Language barrier heads-up.
and to prove the translation isnt fake:
http://translate.google.co.uk/translate?u=http%3A%2F%2Fwww.gamers-globe.dk%2Fnyhed%2F2009%2F11%2Fgran-turismo-5-med-track-editor.html&sl=da&tl=en&hl=en&ie=UTF-8